A conventional drill hole for producing oil from an oil-bearing formation is formed by drilling with a rotary bit driven by a rotating drill pipe which extends through the central opening of a well. A drilling fluid is passed centrally through the drill pipe to remove the cuttings in the excavated area ahead of the bit, in the form of a slurry which is pumped to the surface in the annular space left between the drill pipe and adjacent earth formation. A casing is sunk into the bore hole after drilling.
To drill to great depths, the well may be drilled in steps of successively smaller diameters. At the end of each step, the rotary drill pipe and bit are removed from the hole and a well casing is installed. The original bit is then replaced with a smaller diameter bit to allow it to fit inside the well casing. This use of smaller and smaller bits along with attendant subsequently installed casings results in the formation of a bore hole at the desired depth.
There are a number of disadvantages to the foregoing technique. Firstly, it is inefficient and expensive to continuously operate a rotary drill system and bit at extended depths. Secondly, the casing, typically formed of steel, is expensive and is difficult to install. Thirdly, it is difficult to change the direction of the drilling in the earth formation at radii of less than 1,000-2,000 feet as would be desirable for efficient production of petroleum. Fourthly, the rotation of the drill pipe to which the bit is attached within the casing creates great friction, power loss and wear of both drill pipe and casing.
Also, there is no simple method to make the transition from a drilled vertical bore hole to a horizontal bore hole and to drill along an oil-bearing formation essentially horizontally to permit injection of steam, solvents or other fluids into the formation for enhanced oil recovery from the formation. This capability is particularly required for heavy (high viscosity) oil-bearing formations.
A number of techniques have been attempted to form lateral (essentially horizontal) bore holes from a vertical cased bore hole. In one technique, an oversized vertical bore hole is formed of sufficiently large diameter such that miners may descend to a location near the bottom of the hole from which they can drill horizontal holes by conventional means. This technique is both costly and dangerous, particularly at great depths.
Another technique which has been attempted is known as drainhole drilling. Here, a vertical bore hole is drilled with rotary equipment in a conventional way to form a drill column. A special assembly is attached to the lower end of the drill column, including a pre-formed, non-rotating, curved guide tube and an inner, flexibly jointed, rotatable drive pipe. Then, the drill passes along the curved assembly in a generally lateral direction to drill a substantially horizontal bore hole. A system of this type is described in an article entitled "Drain Space Holes for Tired Old Wells", by D. H. Stormont, Oil and Gas Journal, 53, page 144, Oct. 11, 1954. This system is subject to the disadvantages that there is a high frictional relationship between the curved, flexibly jointed drill pipe and the formation, and it is difficult to form truly horizontal bore holes; instead, downwardly directed bore holes with relatively large turning radii are formed, which are not as desirable as horizontal bore holes. In addition, such bore holes are costly to drill. Also, the cuttings are difficult to remove. Another disadvantage is that the deflected rotating drill pipe tends to wear out due to continuous frictional contact with the formation. Finally, the friction between the deflected rotating drill pipe and the formation limits the extent of the drill penetration.
Another technique has been suggested for driving and lining an underground conduit, primarily in a horizontal direction. There is no suggestion that this system could be employed for drilling oil from an oil-bearing formation or that it could be used to excavate vertically for that purpose. Such system is described in Silverman U. S. Pat. No. 3,422,631. It includes an eversible tube which is driven forwardly under fluid pressure against a bullet-shaped object which is, in turn, moved forwardly through the earth to form a conduit. In this system, there is no suggestion of passing a drilling fluid to, or to form a slurry at, the forward end of the bullet-shaped object to facilitate drilling; in fact, the system is incapable of doing so as it does not provide a channel within the eversible tube for such a fluid. Thus, the soil at the forward end of the bullet-shaped object is compressed by creating great frictional forces which prevent the system from being moved to any considerable distance.
Another system showing a movable eversible tube is disclosed in Masuda U.S. Pat. No. 4,077,610. In this patent, the eversible tube is passed through a preexisting hollow pipe for purposes of passing an article through the pipe. However, nothing in this patent suggests drilling an underground formation in advance of the eversible tube, or of passing a drilling fluid through the eversible tube.